Molecular Approaches to Detection of Bacteria in Critical Care Patients

Abstract

For over 100 years clinical medicine has relied on culture-based techniques to define infections in patients. However, over the past 30 years, it has become clear that culture-independent methods more completely describe both microbial diversity and community dynamics and the importance of such interactions in states of health and disease has been revealed [1, 2], Furthermore, the use of culture-based techniques has clouded our understanding of the pathogenesis of human infections. The concept that one species causes infection by entering the host, defeating the host’s defense system and multiplying to a threshold that allows it to cause injury is probably only applicable for a small subset of microbes, e.g., bioterror agents. The new emerging paradigm in microbial pathogenesis is that many organisms, such as Streptococcus pneumoniae, already exist in bacterial communities of the oro- and nasopharynx of most healthy individuals and that a change in their virulence gene expression and /or an increase in numbers permitting dissemination cause symptoms of infection [3]. The molecular signals that bring about these shifts in pathogen physiology are not fully understood; however, given the importance of bacterial cell-to-cell signaling (quorum sensing) it is possible that shifts in bacterial community composition may lead to emergence and dominance of pre-existing pathogenic species within the community This hypothesis is supported by the finding that within hours of their admission to the intensive care unit (ICU), critically ill patients exhibit dramatic changes in the bacterial communities colonizing their oro- and nasal pharynx [4, 5].